1. Field of the Invention
This invention relates to a power supply apparatus having a plurality of outputs which is suitable as a power supply apparatus for a copier or a printer.
2. Description of the Related Art
In a switching power supply apparatus having a plurality of outputs (hereinafter termed a "multi-output" switching power supply apparatus), in order to stabilize a specific output, a PWM (pulse-width modulation) control circuit determines the "on" period of a switching device of the primary side by performing feedback of the output. Other outputs are controlled by providing a control circuit for the output of a winding of the secondary side, or are output without performing control. For example, when the other outputs have small electric power, the outputs can be easily stabilized by performing a series control using three-terminal regulators or the like. However, when the other outputs have large electric power, a large amount of loss is produced by a series control. Hence, in general, a desired voltage value is obtained by performing chopper control. A magnetic amplifier may also be considered for controlling the other outputs.
(a) However, when performing chopper control, if a DC output is first produced and a chopper-type DC-to-DC converter is provided in the following stage, the number of components increases and the efficiency of the apparatus is reduced. Also when directly chopping the output of a secondary winding, a large amount of loss is produced due to switching.
(b) In a magnetic amplifier, since a saturable reactor is magnetized until a saturated region, a large amount of iron loss is produced in a core material. In addition, since the saturation flux density of a magnetic material generally has a negative temperature coefficient, a very large core is required in order to prevent the phenomenon that at a high temperature and no load, the iron loss of the core increases due to the temperature rise of the core, the temperature of the core thereby rises, the saturation flux density of the core thereby decreases, and therefore voltage control cannot be performed. If the switching frequency is increased for the purpose of providing a small switching power supply apparatus, a dead-angle effect (the phenomenon that an output voltage decreases because the output is hindered for a first predetermined time period of a pulse) caused by the uncontrollable magnetic flux of the saturable reactor and the inductance of the winding increases compared with low-frequency switching. In order to prevent such a phenomenon, it is necessary to reduce the number of turns of the saturable reactor, causing a requirement for a greater core in order to obtain the same saturation flux density. As a result, the merit obtained by high-frequency switching is cancelled. In general, accuracy in a magnetic material (for example, the saturation flux density) is .+-.20% at most. Hence, a larger core must be used in consideration of worst conditions.
Conventional voltage-resonance-type switching regulators have been widely used as devices for generating a high voltage necessary for chargers of copiers, horizontal deflection of CRT's, and the like. Most of these apparatuses are configured such that power supply for a winding of the primary side of a converter transformer is intermitted by switching means, such as a transistor, an FET (field-effect transistor) or the like, to generate a voltage in proportion to the turns ratio at the secondary side.